Treatment of OSA by CPAP gas delivery systems involves the continuous delivery of air (or breathable gas) pressurized above atmospheric pressure to a patient's airways via a conduit and a mask. CPAP pressures of 4 cm H20 to 30 cm H20 are typically used for treatment of sleep disordered breathing due to OSA and/or central apnea, depending on patient requirements.
Treatment pressures for assisted ventilation can range up to 32 cm H20 and beyond, depending on patient requirements.
For either the treatment of OSA or the application of assisted ventilation, the pressure of the gas delivered to patients can be constant level, bi-level (in synchronism with patient inspiration and expiration) or automatically adjusting in level. Throughout this specification the reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of pressure delivery. The prior art method for providing CPAP treatment includes a vent for gas washout of the gas flow. The vent is normally located at or near the mask or in the gas delivery conduit. The flow of gas through the vent is essential for removal of exhaled gases from the breathing circuit. Adequate gas washout is achieved by selecting a vent size and configuration that will allow a minimum safe gas flow at the lowest operating CPAP pressure, which typically can be as low as, around 4 cm H20 for adults and 2 cm H20 in pediatric applications.
Existing vent configurations include single or multiple holes, foam or other diffusers, slots and combinations thereof. A reference herein to a vent may be understood to include a reference to one or more holes, foam or other diffusers, slots or any combination of them.
It is obviously desirable for a CPAP system to have as wide a pressure range as is feasible in order that a standard configuration may adequately provide the unique treatment require by a variety of users. Increasing CPAP pressure results in more gas passing through the vent which in turn creates more noise. Existing prior art vents can produce excessive noise when CPAP pressures are raised above about 4 cm H20. This noise can adversely affect patient and bed-partner comfort. At higher pressures, existing vents are also inefficient as they allow more gas through the vent than is required for adequate exhaust gas washout and thereby require the flow generator to provide more flow than is necessary in order to maintain the required treatment pressure. Further, where treatment gas is being supplied, such as oxygen, surplus treatment gas is vented and thereby wasted unnecessarily. A similar waste occurs where the supplied gas is humidified.
The flow of gas from the gas delivery system through the vent to atmosphere creates noise as the delivered gas, and upon expiration the patient expired gas including C02, passes through the vent to atmosphere. A CPAP system must have a rate of flow through the vent to atmosphere that ensures a clinically undesirable level of expired gas is not retained within the breathing circuit (i.e. within the gas supply conduit and mask chamber). This retention occurs as a result of the exhaled gas not being vented to atmosphere during the exhalation phase of respiration but rather moving down the gas conduit towards the flow generator or accumulating within the mask chamber dead space. An adequate flow of gas to atmosphere may be achieved by selecting the suitable vent size for the clinically desirable pressure treatment range and volume of gas made available by the flow generator to achieve the desired treatment pressure range. Typically this selection involves a compromise being struck between the choice of a vent size that is sufficiently large to achieve an adequate flow rate at the low end of the pressure range and yet cause no greater than an acceptable noise level as the pressure increases through the pressure range. In addition, a large vent which would allow for a generous wash out flow rate at the low end of the pressure range will dictate that the flow generator must have adequate capacity to provide the flow necessary to achieve the desired pressures higher in the pressure range. In short, where the vent size is chosen to deliver a quiet gas wash out flow rate at the higher pressure levels of the pressure range it may be inadequate to allow acceptable wash out flow at the desired lowest end of the pressure range. Also a vent with sufficient size to achieve an adequate wash out flow rate at pressures low in the pressure range tend to generate unacceptable noise at the desired higher end of the pressure range. In addition the choice of a larger vent dictates that the source of gas have capacity to deliver the requisite flow rates for the higher pressure levels and as such the gas source will tend to consume more power and generate louder noise and require additional noise attenuating features so as to keep the total noise within acceptable limits.
Because of the constraints on CPAP system design arising from the vent a choice may be made to limit the lower or upper achievable pressure i.e. for a given upper or lower pressure the delta P between that pressure and the other extreme of the range may be inconveniently constrained. The delta P would be chosen so as to achieve the desired aims of adequate wash out of exhaled gas at the lowest end of the pressure range while capping the noise generated and power consumed at the higher end of the pressure range. Such limitations on the choice of upper or lower pressures and the delta P can seriously confine the usefulness of CPAP system as it is desirable for a standard configuration to have the capacity to deliver the widest pressure range so as to be capable of meeting the clinical requirement of as many users as possible. Achievement of this aim is particularly significant where the CPAP treatment involves the operation of a control algorithm that varies the pressure delivered to the user during the period of treatment (for example on a breath-by-breath basis between two or more pressures or in a more complex manner during the period of treatment). Similarly a computer controlled CPAP system that varies the pressure during the period of treatment in accordance with a control algorithm will include operating parameters which reflect the vent characteristic of the breathing circuit. Because of this it can be undesirable to change from a mask specified for the control algorithm for concern that the new mask should introduce a vent characteristic which is not within the operating parameters of the control algorithm. This inability to change masks because of the accompanying introduction of unknown or incompatible vent characteristics can be adverse to patient compliance with CPAP treatment. This is because a patient may only tolerate CPAP treatment where it is delivered through a particular mask and that mask is incompatible with the prescribed CPAP system control algorithm. Accordingly another aim of the present invention is to provide for a method of configuring and making a vent which can change the vent characteristic of a mask so that the mask may better comply with the operating parameters of a CPAP system control algorithm.
A further aim of the present invention is a method and apparatus for a system of venting which creates a vent having a flow area which varies with changes in pressure occurring at part of or the whole of a CPAP system pressure operating range.
It is known in the art for a CPAP system breathing circuits to include valves that restrict or block venting to atmosphere in given circumstances.
U.S. Pat. No. 5,685,296 to Zdrojkowski discloses a Flow Regulating Valve and Method. In the first embodiment, a rigid insert 52 having a central axial opening 54 is connected to a resilient diaphragm 42. As gas supply pressure increases, the diaphragm 42 flexes toward valve body member 38 and opening 54 moves over a body portion 70 of regulating pin 62, thereby decreasing the flow area between opening 54 and regulating pin 62 and maintaining a relatively constant gas flow rate even at the higher gas pressure. In additional embodiments, gas supply pressure is used to move flexible diaphragms 42′ and 42″ toward respective valve body walls, thereby decreasing the gas flow areas between the respective diaphragms and the valve body walls and preventing higher gas flows at higher gas pressures.
U.S. Pat. No. 6,006,748 to Hollis discloses a Vent Valve Apparatus which is adapted to progressively restrict a flow area of a washout vent as the pressure of the gas supply increases. In two embodiments disclosed therein, a flexible diaphragm 20 sensitive to the pressure of the gas supply is connected by a rigid wire rod 23 to a conical plug 18 positioned in a conical orifice 15. As the pressure of the gas supply increases, the diaphragm 20 bulges outward. This moves the rod 23 and conical plug 18 such that the conical plug 18 is drawn into the orifice 15, thereby decreasing the flow area of the vent between the plug 18 and orifice 15 and restricting the flow of gasses through the vent. In a third embodiment, an aerodynamic wing 30 replaces the diaphragm 20 and moves the conical plug in relation to gas flow past the aerodynamic surfaces of the wing.
While each of these references discloses embodiments that restrict gas flow as the pressure of the gas supply increases, there is a desire to provide a flow regulation vent that is simpler and cheaper to manufacture while providing the opportunity to have the flow through the vent vary as the pressure varies in a manner that is not limited to achieving a constant flow rate.
These valves are generally known as non-rebreath or anti-asphyxia valves. An example of a non-rebreath valve is U.S. Pat. No. 5,438,981 to Starr et al. for an Automatic Safety Valve And Diffuser For Nasal And/Or Oral Gas Delivery Mask which includes a valve element 32 that can pivot between a first position and a second position to allow inflow into a mask from either a gas flow generator or the atmosphere. The safety valve does not restrict gas flow as the pressure of the gas supply increases.
Other examples of safety valves can be found in U.S. Pat. Nos. 5,896,857, 6,189,532 (Hely/Lithgow assigned to ResMed Limited) and WO 00/38772 (Walker et. al assigned to ResMed Limited).
An embodiment of the vent of the present invention could also serve as a non-rebreath or antiasphyxia valve.